In copending application of William H. Lake, Ser. No. 609,505, filed of even date, entitled "Arc Discharge Lamp With Starter Electrode Voltage Doubling" and which is assigned like this application, a lamp is disclosed having improved voltage doubling starting means which reduces the burden on the ballast and substantially eliminates electrolysis during operation. The lamp comprises a vitreous arc tube having main electrodes at opposite ends and a starter electrode adjacent one of them, an ionizable fill in the arc tube, and a voltage doubling circuit comprising a diode and a capacitor connected in series across input terminals. The circuit is poled to generate a positive potential at the junction of diode and capacitor which is coupled through a resistor to the starter electrode. The diode builds up a charge across the capacitor on one half-cycle which is added to the line voltage on the opposite half-cycle and approximately doubles the voltage provided between the starter and the selected main electrode. Voltage doubling may be provided across the gap from starter to remote main electrode, or across the gap from starter to adjacent main electrode to improve starting in different types of lamps.
The mount frame which supports the arc tube within the outer envelope is connected to the junction of the diode and capacitor in the voltage doubling circuit in order to receive a positive potential which reduces sodium loss by electrolysis through the arc tube walls. Such construction requires that at least the major portion of the mount frame be isolated from the main current inleads into the outer envelope or jacket of the lamp. If the diode and capacitor are mounted in the base of the lamp, a three-lead stem for the jacket is mandatory. This type of stem increases manufacturing costs in terms of materials, labor and tooling. It may also reduce reliability by increasing the likelihood of arc over at the stem leads during hot restart or upon arc tube failure.
By installing the diode and capacitor inside the jacket, a two-lead stem can suffice if an independent support wire for the frame is provided. Alternatively a glass bead insulator can be used to support the frame from one of the inleads while simultaneously insulating it. However these arrangements are also relatively costly and in the case of a glass bead insulator the mechanical strength may be insufficient.